Subject: BSSE in cyclophanes?

From: "Michael D. Bartberger" <[email-masked]>
Date: Fri, 20 Jun 1997 01:59:39 -0400 (EDT)

Greetings:

I am currently investigating the thermodynamics of formation for a series
of substituted para-cyclophanes, of the general type

                         CH2---Ph---CH2
                         |           |
                         |           |
                         CH2---Ph---CH2

                                                     /CH=CH\ 
as formed from their xylylene "monomers":   2   CH2=C       C=CH2
                                                     \CH=CH/
(pardon the ASCII artwork!)

Methodology so far has been as follows: geometry optimizations and
frequencies/ZPE determination at RHF/3-21G (I'd like to go larger but I
have other substituted systems where the number of heavy atoms gets quite
large)  with single-point energies at MP2/6-31G* and B3LYP/6-31G* on the
RHF/3-21G geometries.

I've been worrying about BSSE.  As far as I've determined from
following various threads of discussion on this list, BSSE manifests
itself when determining energetics of interacting vs. isolated "monomers",
where a superposition of molecule A's basis functions onto molecule B (and
vice versa) occurs.  I can see the ramifications of this, in, say, a case
such as the acetic acid dimer, for instance.

Where I'm unsure is when the "dimer" is formally bound, such as above.
Should one account for BSSE effects in such a case?  Surely some
superposition of basis functions onto adjacent/nearby nuclei occurs (?)-
if so, then it most certainly does so in just about any other organic
system (again, ??)   :)  Pardon me, I don't mean to get too
philosophical.......and I hope the question is not ridiculous.

The burning question is the following: where does one draw the line when
deciding to incorporate BSSE effects?  Intuitively, I'd imagine one does
not include them in a case like this, a formally it (the dimer) is
a single molecule........  otherwise, BSSE would become a factor in, for
example, [4+2] or other cycloadditions, or for that matter, any other
reaction where bonds are being formed.

Am I thinking too hard about this????   :)  Sorry, it's late.....

I'll certainly summarize responses-  I'd greatly appreciate any insight 
the community might be able to lend.

Thanks very much!

Best Regards,
-Michael
________________________________________________________________________________

 Michael D. Bartberger         [email-masked]        TEL: (352) 392-3580
 Department of Chemistry       [email-masked]         FAX: (352) 846-0296
 University of Florida
 Gainesville, FL  32611
 USA
________________________________________________________________________________